1. Field of the Invention
The present invention relates to a turbo vacuum pump.
2. Description of the Related Art
A conventional turbo vacuum pump in which atmospheric pressure is maintained in its outlet port is proposed in, for example, Japanese Patent Unexamined Publication No. 62-113887.
In this conventional turbo vacuum pump, a first impeller and a diffuser fixing plate are arranged in the axial direction, and second impellers and fixing plates are arranged alternately. For this reason, the diffuser fixing plate and the fixing plates must be formed as two-piece fitting structures.
In general, in order to attain satisfactory pump performance, the above turbo vacuum pump must be made to maintain a predetermined small gap between the impellers and the respective fixing plates, particularly between the second impellers and the fixing plates. However, in the case of the two-piece fitting structures of the fixing plates, the processing accuracy is difficult to maintain because of the complex construction, and the above small gap for the pump performance may not be insured.
In the proposed type described in, for example, Japanese Patent Unexamined Publication No. 62/29796, radial blades are in general employed and compressing operation is effected by forming a swirl by the action of, primarily, centrifugal force.
In the other proposed type described in Japanese Patent Unexamined Publication No. 63/147989, each blade is in general a forward arc blade, and a flow is deflected by means of the blades to form a swirl, thereby achieving compressing operation.
Although the prior art arrangement disclosed in Japanese Patent Unexamined Publication No. 62/29796 effects compressing operation by sufficiently utilizing centrifugal force, the function of the blades, that is, the function of deflecting a flow is not taken into account. Accordingly, this prior art has the problem that the compression ratio of the centrifugal-flow pump cannot be increased so as to achieve high performance.
Although the prior art arrangement disclosed in Japanese Patent Unexamined Publication No. 63/147989 effects compressing operation by sufficiently utilizing the function of blades, the utilization of centrifugal force is not taken into account. Accordingly, this prior art has the problem that the compression ratio of the centrifugal-flow pump cannot be increased so as to achieve high performance.